Fuel cells are highly efficient, safe in operations, and low in pollution. Therefore, they are applied in a variety of fields, such as electric power, industry, transportation, aeronautics, military, and the like. A fuel cell is a power generating device, which can continuously and directly converting chemical energy into electrical energy. When a fuel cell is in operation, a fuel gas (such as hydrogen gas) and a combustion promoter (such as oxygen gas) are delivered to the anode and cathode of the fuel cell, respectively. Oxidation and reduction then take place to convert chemical energy into electrical energy.
The structure of a conventional fuel cell unit is substantially consisted of an anode plate, a cathode plate, and a solid electrolyte film interposed between the anode plate and the cathode plate, and is referred to as a battery cell. However, in practical uses, multiple battery cells may be connected in series, so as to achieve a greater output voltage. Adjacent fuel cell units have a common electrode plate, which serves as the anode and cathode of the two adjacent fuel cell units, respectively. Thus, the electrode plate is referred to as a bipolar plate.
Currently, in the structure of a bipolar plate for a fuel cell, a polymeric material is overlaid on a stainless steel substrate by spray-coating, and the polymeric material is bonded to the substrate as a result of pyrolysis. At least 90% of conductive graphite is added to the polymeric material to block corrosion and oxidation caused by the external environment to the stainless steel substrate, and to confer conductivity to the stainless steel substrate. However, the conductive protective film is formed by spray-coating, whereby the solvent in the polymeric material in the film evaporates during pyrolysis, such that numerous small air bubbles are generated in the conductive protective film finally formed. This causes the conductive protective film to have poor density. Further, the small air bubbles serve as channels for the infiltration of the acidic solution of a fuel cell into the bipolar plate. Moreover, film formation on the overlaid layer formed by a coating process is ineffective at a specific angle on the flow field structure, because the flow field structure at the surface of the metal substrate have a complex structure along the horizontal and vertical directions. Hence, multiple layers of polymeric materials must first be coated on the surface of the stainless steel substrate, and then conducting a number of pyrolytic processes, in order to avoid the occurrence of the aforesaid problems. Nevertheless, this makes the processing procedures for the bipolar plate be too complicated, and also makes the production cost be too expensive.
Further, a gas separator, which uses a tin paste to bond a graphite layer to a stainless steel base, has been developed. Specifically, heating and pressurizing are utilized to bond the graphite layer to the stainless steel substrate via an interposed tin layer. However, the tin-containing gas separator would poison a fuel cell and the graphite layer has hetero junction with the tin layer, such that delamination occurs among the layers due to poor strengths. Moreover, acid solution may permeate the graphite layer to corrosion the tin layer and poison MEA to make PEM fuel cell break down.
In addition, a bipolar plate comprised of a resin material has been developed which involves the preparation of a plurality of molded sheets containing carbon materials, and then press-fitting of the molded sheet to obtain the bipolar plate. However, the problem of poor air tightness of the bipolar plate results from the control of the composite carbon board to extreme thinness.